Rolling-contact shaft member

ABSTRACT

A rolling-contact shaft member, which is made of high-carbon steel and whose outer peripheral surface serves as a rolling-contact surface that rolling-contacts a mating material, includes: a carbonitrided layer having a carbon concentration of 1.1 to 1.6 wt % and a nitrogen concentration of 0.05 to 0.6 wt % in the range from the surface to the depth of 10 μm. The rolling-contact shaft member has a Vickers hardness of 700 to 840 HV at the outer peripheral surface and has a Vickers hardness of 600 HV or less in its central portion. A maximum value of an absolute value of a gradient of a change in the Vickers hardness from the outer peripheral surface to the central portion is 100 to 340 HV/mm.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2016-090736 filed on Apr. 28, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to rolling-contact shaft members that rolling-contacts a mating material.

2. Description of the Related Art

One type of rocker arms that are used in valve systems of automotive engines etc. is a roller rocker arm including a rocker roller. In the roller rocker arm, the rocker roller that contacts a cam is rotatably attached to the rocker arm via a shaft. In such a rocker arm, the inner peripheral surface of the rocker roller rolling-contacts the outer peripheral surface of the shaft. In order to prevent breakage of the shaft and prolong the rolling life of the shaft, it is proposed to reform a steel material in a production process of the shaft. For example, Japanese Patent Application Publication No. 2008-63603 (JP 2008-63603 A) proposes a method for producing a shaft which includes preparation of a member made of steel, carbonitriding, cooling, induction hardening, tempering, and finishing.

However, since the method described in JP 2008-63603 A has many heat treatment processes, manufacturing cost is high and it takes a long time to produce a shaft. The inventors intensively studied to prolong the rolling life of shaft members (rolling-contact shaft members) that rolling-contact a mating material and completed the present invention based on a new idea.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a rolling-contact shaft member that has a prolonged rolling life and that can be produced at low cost, by attaining predetermined hardness distribution from the surface of the rolling-contact shaft member to the inside thereof.

According to one aspect of the present invention, a rolling-contact shaft member, which is made of high-carbon steel and whose outer peripheral surface serves as a rolling-contact surface that rolling-contacts a mating material, includes: a carbonitrided layer having a carbon concentration of 1.1 to 1.6 wt % and a nitrogen concentration of 0.05 to 0.6 wt % in a range from a surface to a depth of 10 μm. The rolling-contact shaft member has a Vickers hardness of 700 to 840 HV at the outer peripheral surface and has a Vickers hardness of 600 HV or less in its central portion, and a maximum value of an absolute value of a gradient of a change in the Vickers hardness from the outer peripheral surface to the central portion is 100 to 340 HV/mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1A is a partial front view of a roller rocker arm using a rolling-contact shaft member according to an embodiment of the present invention;

FIG. 1B is a sectional view taken along line A-A in FIG. 1A;

FIG. 2A is a perspective view of the rolling-contact shaft member shown in FIGS. 1A, 1B;

FIG. 2B is a sectional view taken along line B-B in FIG. 2A;

FIG. 3 is a graph schematically showing hardness distribution in a section of the rolling-contact shaft member shown in FIGS. 2A, 2B;

FIG. 4 is a flowchart illustrating a method for producing the rolling-contact shaft member shown in FIGS. 2A, 2B;

FIG. 5 is a schematic illustration showing an example of a heat treatment device that is used to produce the rolling-contact shaft member shown in FIG. 2A, 2B;

FIG. 6 is a diagram showing heat treatment conditions in Example 1;

FIG. 7 is a diagram showing heat treatment conditions in Comparative Example 1;

FIG. 8 is a diagram showing heat treatment conditions in Comparative Example 2;

FIG. 9 is a diagram showing heat treatment conditions in Comparative Example 3; and

FIG. 10 is a diagram showing heat treatment conditions in Comparative Example 4.

DETAILED DESCRIPTION OF EMBODIMENTS

A rolling-contact shaft member according to an embodiment of the present invention will be described below with reference to the accompanying drawings. The rolling-contact shaft member according to the embodiment of the present invention can be used as a shaft (rocker roller shaft) that rotatably supports a rocker roller included in a rocker arm. FIG. 1A is a partial front view of a roller rocker arm using a rolling-contact shaft member according to an embodiment of the present invention. FIG. 1B is a sectional view taken along line A-A in FIG. 1A. FIG. 2A is a perspective view of the rolling-contact shaft member shown in FIGS. 1A, 1B. FIG. 2B is a sectional view taken along line B-B in FIG. 2A.

As shown in FIGS. 1A and 1B, a roller rocker arm 1 includes a rocker arm body 10, a rocker roller shaft (hereinafter sometimes simply referred to as the shaft) 11, and a rocker roller 12 that contacts a cam 2. The rocker arm body 10 includes a pair of bifurcated wall portions 10A, 10B in its one end. The wall portions 10A, 10B have respective through holes 10C formed coaxially. The shaft 11 is fitted in the through holes 10C and is fixed to the wall portions 10A, 10B. The rocker roller 12 is placed between the wall portions 10A, 10B and is rotatably attached to the shaft 11. In this rocker roller arm 1, an inner peripheral surface 12A of the rocker roller 12 rolling-contacts an outer peripheral surface 11A of the shaft 11. Both ends 11B, 11C of the shaft 11 are clinched, whereby the shaft 11 is fixed to the wall portions 10A, 10B of the rocker arm body 10.

The shaft 11 is a columnar member as shown in FIGS. 2A, 2B. The shaft 11 is made of high-carbon steel (carbon content of 0.7 to 1.1 wt %). The shaft 11 has a carbonitrided layer 11D in its surface layer portion. The carbonitrided layer 11D is formed by a carbonitriding treatment. Since the shaft 11 is made of high-carbon steel, the carbonitrided layer 11D is formed in the surface layer portion and the shaft 11 is caused to have predetermined hardness distribution from the surface toward the inside as described below. This can effectively prolong the rolling life. However, for example, if low-carbon steel is used, the rolling life can hardly prolonged even if the carbonitrided layer 11D is formed at the surface of the shaft and the shaft is caused to have predetermined hardness distribution from the surface toward the inside. High-carbon steel is therefore used for the shaft 11. Any high-carbon steel having a carbon content of 0.7 to 1.1 wt % may be used. Examples of such high-carbon steel include high carbon chromium bearing steels such as SUJ2 and SUJ3.

The shaft 11 has the carbonitrided layer 11D in its surface layer portion. The shaft 11 therefore contains a large amount of solid solution of carbon and nitrogen near its surface layer portion. Accordingly, the shaft 11 has large compressive residual stress at its surface and is less likely to be broken by rolling contact with the rocker roller 12. The rolling life of the shaft 11 can thus be prolonged.

The carbonitrided layer 11D has a carbon concentration of 1.1 to 1.6 wt % in the range from its surface to the depth of 10 μm. If the carbon concentration is less than 1.1 wt %, the shaft 11 does not have sufficient strength in its surface structure and it is difficult to prolong the rolling life of the shaft 11. If the carbon concentration is more than 1.6 wt %, the shaft 11 contains a larger amount of coarse carbides in its surface structure, and therefore has a larger number of starting points for cracks. In this case, the shaft 11 is more easily broken and it is difficult to prolong the rolling life of the shaft 11.

The carbonitrided layer 11D has a nitrogen concentration of 0.05 to 0.6 wt % in the range from its surface to the depth of 10 μm. If the nitrogen concentration is less than 0.05 wt %, the rolling life of the shaft 11 can be hardly prolonged. If the nitrogen concentration is more than 0.6 wt %, the shaft 11 contains a larger amount of retained austenite. Accordingly, the shaft 11 has lower surface hardness and thus has shorter rolling life.

The shaft 11 has a Vickers hardness of 700 to 840 HV at its outer peripheral surface 11A. If the Vickers hardness at the outer peripheral surface 11A is less than 700 HV, the outer peripheral surface 11A is too soft to serve as a surface that rolling-contacts the rocker roller 12, and a long rolling life cannot be expected. If the Vickers hardness at the outer peripheral surface 11A is more than 840 HV, the shaft 11 has lower shock resistance. The Vickers hardness at the outer peripheral surface 11A is preferably 720 to 840 HV. This can significantly prolong the rolling life of the shaft 11. The Vickers hardness at the outer peripheral surface 11A is measured in the middle portion in the longitudinal direction of the shaft 11.

The shaft 11 has a Vickers hardness of 600 HV or less in its central portion M. This can increase the rolling life of the shaft 11. The Vickers hardness in the central portion M is preferably 550 HV or less. This ensures that the shaft 11 has a sufficient hardness difference between the outer peripheral surface 11A and the central portion M. The rolling life of the shaft 11 can thus be significantly prolonged. The Vickers hardness in the central portion M is preferably 350 HV or more. If the Vickers hardness in the central portion M is less than 350 HV, the shaft 11 may not have sufficient crushing strength.

FIG. 3 is a graph schematically showing hardness distribution in a section of the shaft 11 shown in FIGS. 2A, 2B. FIG. 3 shows hardness distribution from the outer peripheral surface 11A to the central portion M of the shaft 11. The maximum value Smax of the absolute value of the gradient S of the change in Vickers hardness from the outer peripheral surface 11A to the central portion M of the shaft 11 (hereinafter the maximum value Smax is sometimes referred to as the “maximum gradient Smax”) is 100 to 340 HV/mm. As shown in FIG. 3, Vickers hardness decreases from the outer peripheral surface 11A toward the central portion M of the shaft 11. In this case, it is important for the gradient S (HV/mm) of the change in Vickers hardness to meet specific requirements in order to prolong the rolling life of the shaft 11. As described above, it is important for the maximum gradient Smax of the change in Vickers hardness to be 100 to 340 HV/mm. With the maximum gradient Smax being in this range, the life of the shaft 11 can be prolonged.

If the maximum gradient Smax is less than 100 HV/mm, the shaft 11 does not have a portion where Vickers hardness changes significantly. The shaft 11 therefore has small compressive residual stress, and the rolling life of the shaft 11 cannot be prolonged. If the maximum gradient Smax is larger than 340 HV/mm, the shaft 11 has large tensile residual stress and thus does not have sufficient strength. The shaft 11 can therefore become unusable quickly.

The maximum gradient Smax is preferably 170 to 340 HV/mm. The maximum gradient Smax in this range is suitable for prolonging the life of the shaft 11.

In the present invention, the gradient S (HV/mm) of the change in Vickers hardness means the slope in the graph plotting the change in Vickers hardness from the outer peripheral surface 11A to the central portion M of the shaft 11 as shown in FIG. 3. The slope (gradient S) usually has a negative value. The gradient S can have various values according to the depth from the surface. Of the gradient S that varies according to the depth, the gradient S having the largest absolute value is the maximum value Smax of the absolute value of the gradient S (maximum gradient Smax).

In the present embodiment, the gradient S can be obtained by the following method. In the section of the shaft 11, Vickers hardness is measured at regular intervals at a predetermined pitch (0.1 mm) from the outer peripheral surface 11A to the central portion M. Thereafter, a distribution diagram of Vickers hardness with respect to the depth is created based on the measured values, and the maximum gradient Smax is calculated based on this distribution diagram. At this time, the maximum gradient Smax can be determined by obtaining gradients between two measurement points and comparing the gradients with each other.

In the shaft 11, it is preferable that x/d be 0.05 to 0.3, where x represents the position (depth from the outer peripheral surface 11A of the shaft 11) at which the shaft 11 has the maximum gradient Smax, and d represents the depth of the central portion M. If x/d is less than 0.5, Vickers hardness significantly changes near the outer peripheral surface 11A of the shaft 11. Performance of the shaft 11 therefore may not be stable. If x/d is more than 0.3, the shaft 11 has the maximum gradient Smax at a deep position and has small compressive residual stress at its surface. Accordingly, the life of the shaft 11 may not be able to be prolonged. The depth d of the central portion M corresponds to the radius of the shaft 11.

The shaft 11 has a Vickers hardness of 210 to 300 HV at its end faces 11B, 11C. The end faces 11B, 11C of the shaft 11 can therefore be more easily plastically deformed as compared to the outer peripheral surface 11A of the shaft 11. Accordingly, as shown in FIGS. 1A, 1B, the shaft 11 can be fixed to the rocker arm body 10 by clinching the end faces 11B, 11C of the shaft 11.

Although the size of the shaft 11 is not particularly limited, it is preferable that the diameter of the shaft 11 be 3.5 mm or more. If the diameter of the shaft 11 is less than 3.5 mm, it is not easy to achieve the above hardness distribution from the outer peripheral surface 11A to the central portion M of the shaft 11. It is preferable that the diameter of the shaft 11 be 30 mm or less. If the diameter of the shaft 11 is larger than 30 mm, the rolling life of the shaft 11 may not be prolonged so much.

This shaft 11 has predetermined hardness distribution from the outer peripheral surface 11A toward the inside, and has low hardness at its end faces 11B, 11C. The shaft 11 is therefore a shaft that has a long rolling life and that can be attached by clinching.

For example, the shaft 11 can be produced by a method which includes: (a) carbonitriding and quenching a shaft-shaped workpiece; (b) tempering the carbonitrided, quenched workpiece by immersing the workpiece in a cooling liquid and inductively heating the workpiece immersed in the cooling liquid; (c) finishing the tempered workpiece, and in which in (b), the workpiece is inductively heated with its end faces being covered by a jig.

The method for producing the shaft 11 will be described below. FIG. 4 is a flowchart illustrating a method for producing the rolling-contact shaft member (shaft) shown in FIGS. 2A, 2B.

(1) First, a steel material made of high-carbon steel is subjected to rough processing such as forging or machining. Namely, preprocessing is performed to produce a formed material (workpiece) W1 of the shaft 11 having a columnar shape (step S1 in FIG. 4).

(2) Next, the workpiece W1 thus produced is carbonitrided and quenched to produce a workpiece W2 having a carbonitrided layer at its surface (step S2 in FIG. 4). For example, the workpiece W1 is carbonitrided and quenched under the following conditions. The workpiece W1 is heated and held at 850 to 900° C. for a predetermined time in an carbonitriding atmosphere having a carbon potential of 1.1 to 1.3% and an ammonia gas concentration of 2 to 7 vol %, and is then rapidly cooled to a predetermined temperature.

It is preferable that the carbon potential of the carbonitriding atmosphere be 1.1 to 1.3%. The use of the carbonitriding atmosphere with a carbon potential of 1.1% or more allows the workpiece W2 to have sufficient hardness at its surface. The use of the carbonitriding atmosphere with a carbon potential of 1.3% or less can restrain formation of an excessively carburized structure at the surface of the workpiece W2.

It is preferable that the ammonia gas concentration of the carbonitriding atmosphere be 2 to 7 vol %. The use of the carbonitriding atmosphere with an ammonia gas concentration of 2 vol % or more can restrain formation of an excessively carburized structure and can increase resistance to temper softening that is caused by the subsequent tempering. The use of the carbonitriding atmosphere with an ammonia gas concentration of 7 vol % or less can prevent embrittlement due to excessive nitriding.

It is preferable that the temperature at which the workpiece W1 is heated and held in the carbonitriding atmosphere be 850 to 900° C. Heating and holding the workpiece W1 at a temperature of 850° C. or more can form a sufficient carbonitrided layer. Heating and holding the workpiece W1 at a temperature of 900° C. or less can restrain introduction of an excessive amount of carbon into the workpiece W1 and thus restrain formation of an excessively carburized structure in the workpiece W2 produced by carbonitriding and quenching the workpiece W1, and can restrain precipitation of coarse carbides and carbonitrides.

Although the time for which the workpiece W1 is heated and held is not particularly limited, it is preferable that the workpiece W1 be heated and held for 4 hours or more. This can achieve a sufficient carburizing depth. For example, the rapid cooling after heating is performed by oil quenching in a quenching oil bath. The oil bath temperature of quenching oil is normally 60 to 180° C. The above conditions for carbonitriding and quenching can be changed as desired according to the size of the workpiece W1 etc.

(3) Subsequently, the workpiece W2 produced by carbonitriding and quenching the workpiece W1 is tempered to produce a workpiece W3 (step S3 in FIG. 4). For example, this tempering treatment is performed by immersing the workpiece W2 in a cooling liquid, inductively heating the workpiece W2 immersed in the cooling liquid for a predetermined time, and then cooling the workpiece W2 in the cooling liquid. For example, the workpiece W2 may alternatively be forcibly cooled by air, or by being allowed to cool in still air, etc. At this time, the workpiece W2 has its both end faces covered by a jig, and the inductive heating is performed in this state.

In this tempering treatment, the tempering temperature of the outer peripheral surface of the workpiece W2 (outer peripheral surface temperature) can be made lower than the tempering temperature of the central portion of the workpiece W2 (internal temperature). This can restrain a decrease in retained austenite in the tempering treatment and can increase compressive residual stress. It is preferable that the difference between the outer peripheral surface temperature and the internal temperature (the internal temperature minus the outer peripheral surface temperature) be 40 to 600° C. Adjusting the temperature difference in this range can achieve predetermined hardness distribution from the outer peripheral surface toward the central portion of the workpiece W2. If the temperature difference is larger than 600° C., the workpiece W2 may crack.

In the tempering treatment, it is preferable that the tempering time be 20 seconds or less. This can apply sufficient compressive residual stress to the workpiece W2. The tempering time is more preferably 18 seconds or less. The tempering time is preferably 2 seconds or more, and more preferably 3 seconds or more, because temperature unevenness can be restrained and product quality can be improved. In the present embodiment, the “tempering time” refers to the time during which a current is applied in the induction heating.

Specifically, it is preferable that the temperature of the tempering treatment be adjusted so that the outer peripheral surface temperature is 170 to 290° C. and the internal temperature is 320 to 715° C. It is preferable that the outer peripheral surface temperature be 275° C. or less, because such an outer peripheral surface temperature is more suitable for improving the rolling life. It is preferable that the internal temperature be more preferably 365° C. or more, and even more preferably 450° C. or more, because such an internal temperature is suitable for ensuring shock resistance. It is more preferable that the internal temperature be 575° C. or less, because such an internal temperature is suitable for ensuring crushing strength. It is even more preferable that the internal temperature be 450 to 575° C. in order to ensure high shock resistance and high crushing strength. The outer peripheral surface temperature and the internal temperature can be measured with a K-type thermocouple. The tempering temperature can be adjusted by the frequency and output of the induction heating, the tempering time, etc.

For example, the tempering treatment is performed by using a heat treatment device described below. FIG. 5 is a schematic illustration showing an example of a heat treatment device that is used to produce the rolling-contact shaft member shown in FIG. 2A, 2B. A heat treatment device 100 shown in FIG. 5 includes a treatment tank 101, a holding jig 102, an induction heating coil 103, a cooling liquid 105, and a supply pipe 106. The treatment tank 101 is a tank in which the workpiece W2 resulting from the quenching is placed and tempered. The holding jig 102 holds the workpiece W2. The induction heating coil 103 placed in the outer periphery of the treatment tank 101 inductively heats the workpiece W2. The cooling liquid 105 is a cooling medium that is stored in the treatment tank 101 to cool the workpiece W2. The supply pipe 106 supplies the cooling liquid 105 into the treatment tank 101.

The treatment tank 101 is a bottomed cylindrical container that can store the cooling liquid 105. This container is made of an electrically insulating ceramic or an electrically insulating synthetic resin. This can restrain heating of the heat treatment device 100 itself. The size of the treatment tank 101 can be determined as desired according to the size of the workpiece W2 etc. The cooling liquid 105 is stored in the treatment tank 101. The treatment tank 101 has a drain port 108 that discharges an excess cooling liquid 105 to the outside of the treatment tank 101.

The holding jig 102 is a member that holds the workpiece W2. The holding jig 102 is made of heat-resistant cement etc. The holding jig 102 holds the workpiece W2 such that the longitudinal direction of the workpiece W2 extends in the vertical direction. The holding jig 102 includes a lower jig 102A and an upper jig 102B. The lower jig 102A holds the workpiece W2 so as to cover a lower end face W2B of the workpiece W2. The upper jig 102B holds the workpiece W2 so as to cover an upper end face W2C of the workpiece W2.

The lower jig 102A includes a circular plate-shaped jig body 112 and a columnar support shaft 113. The jig body 112 holds the workpiece W2 so as to cover the lower end face W2B of the workpiece W2. The support shaft 113 is provided on the lower surface of the jig body 112, and the lower jig 102A is attached to the bottom portion of the treatment tank 101. The jig body 112 has a shallow recessed portion 112 a in its upper surface. The recessed portion 112 a is formed to have substantially the same shape as the lower end face W2B of the workpiece W2. The lower jig 102A holds the workpiece W2 with the lower end face W2B of the workpiece W2 being fitted in the recessed portion 112 a. The upper jig 102B includes a circular plate-shaped jig body 122 and a columnar support shaft 123. The jig body 122 holds the workpiece W2 so as to cover the upper end face W2C of the workpiece W2. The support shaft 123 is provided on the upper surface of the jig body 122, and the upper jig 102B is attached to a lid portion (not shown) of the heat treatment device 100. The jig body 122 has a shallow recessed portion 122 a in its lower surface. The recessed portion 122 a is formed to have substantially the same shape as the upper end face W2C of the workpiece W2. The upper jig 102B holds the workpiece W2 with the upper end face W2C of the workpiece W2 being fitted in the recessed portion 122 a.

With such a holding jig 102, the workpiece W2 can be held at a predetermined position in the treatment tank 101 so as to be immersed in the cooling liquid 105. In the case where the workpiece W2 is held by the holding jig 102, not only both end faces of the workpiece W2 but also the portions of the outer peripheral surface W2A of the workpiece W2 which are located near these end faces are covered by the holding jig 102, and the tempering treatment is performed in this state.

The induction heating coil 103 is placed outside the treatment tank 101. The induction heating coil 103 is a helical coil whose inside diameter is larger than the outside diameter of the treatment tank 101. The heat treatment device 100 can inductively heat the workpiece W2 to a desired temperature by supplying a current to the induction heating coil 103. The induction heating coil 103 may be placed inside the treatment tank 101.

The cooling liquid 105 may be any liquid that can cool the surface of the workpiece W2. Examples of the cooling liquid 105 include water, oil, a water-soluble polymer, etc. Examples of the oil include quenching oil etc. Examples of the water-soluble polymer include water-soluble quenchants such as polyalkylene glycol (PAG). The water-soluble polymer can be used as an aqueous solution. In this case, the amount of water-soluble polymer that is added to water can be determined as desired according to the kind of water-soluble polymer etc. The cooling liquid 105 is preferably a cooling liquid with a high heat transfer coefficient, and more preferably a cooling liquid that is easy to handle, because the surface of the workpiece W2 can be efficiently cooled.

The supply pipe 106 serves to supply the cooling liquid 105 into the treatment tank 101. The heat treatment device 100 includes a plurality of the supply pipes 106. Each supply pipe 106 has a plurality of injection nozzles 106 a in its intermediate portion and injects the cooling liquid toward the workpiece W2 through the injection nozzles 106 a.

The supply pipes 106 can thus not only supply the cooling liquid 105 into the treatment tank 101 but also stir the cooling liquid 105 stored in the treatment tank 101 as they supply the cooling liquid 105 through the injection nozzles 106 a. Each supply pipe 106 has a flow regulating valve and a pressure regulating valve (both not shown). The conditions for supplying the cooling liquid 105 can thus be adjusted.

In the heat treatment device 100, the cooling liquid 105 supplied through the supply pipes 106 is stored in the treatment tank 101 and excess cooling liquid 105 is discharged through the drain port 108. The heat treatment device 100 may have a circulating path (not shown) that supplies the discharged cooling liquid 105 back into the treatment tank 101.

Although not shown in the figure, the heat treatment device 100 further includes necessary members such as a power supply required for induction heating, a matching unit, a temperature regulating member that controls the temperature of a cooling agent etc. The heat treatment device 100 may include a mechanism that rotates the workpiece W2 held by the holding jig 102 about an axis.

The tempering treatment using this heat treatment device 100 is performed by placing the workpiece W2 in the treatment tank 101 such that the workpiece W2 is held by the holding jig 102 and inductively heating the workpiece W2 immersed in the cooling liquid 105, as described above. Accordingly, the portions of the workpiece W2 which are covered by the holding jig 102, namely both end faces W2B, W2C of the workpiece W2 and the portions of the outer peripheral surface W2A of the workpiece W2 which are located near the end faces W2B, W2C, are tempered at a high temperature because these portion of the workpiece W2 do not contact the cooling liquid 105 during heating. The portion of the outer peripheral surface W2A of the workpiece W2 which is not covered by the holding jig 102 is tempered at a lower temperature than the portions covered by the holding jig 102 because the portion not covered by the holding jig 102 is in contact with the cooling liquid 105 during heating.

Accordingly, performing the tempering treatment using this heat treatment device 100 allows the workpiece W2 to have low hardness in its end faces and portions near the end faces so that clinching can be performed, and to have high hardness in the most part of its outer peripheral surface.

The frequency and output of the induction heating can be determined as desired according to the shape and size of the workpiece W2, the cooling capacity of the cooling liquid, etc. For example, the frequency may be 0.3 to 3 kHz and the output may be 50 to 300 kW. Adjusting the frequency and the output in such a range allows the shaft 11 to have the above hardness distribution from the outer peripheral surface 11A to the central portion M. For example, the cooling liquid 105 is supplied through the injection nozzles 106 a at 20 to 80 L/min during the induction heating, although the present invention is not particularly limited to this. The temperature of the cooling liquid 105 to be supplied varies depending on the cooling capability of the cooling liquid etc. However, for example, if the cooling liquid 105 is a water-soluble polymer, the temperature of the cooling liquid 105 is 20 to 40° C.

(4) Lastly, the workpiece W3 resulting from the tempering treatment is subjected to finishing such as polishing (step S4 in FIG. 4). Through the above treatments, the shaft 11 can be produced with reduced cost.

The present invention is not limited to the above embodiment and can be modified as desired within the scope of the claims. The rolling-contact shaft member according to the embodiment of the present invention is not limited to the shaft of the rocker roller. The rolling-contact shaft member may be any shaft member that rolling-contacts a mating material. For example, the rolling-contact shaft member can be suitably used for inner shafts (inner rings) of needle roller bearings.

The tempering treatment using the heat treatment device 100 may be performed with only one end face (e.g., only the lower end face W2B) of the workpiece W2 being held by the holding jig 102A. In this case, only one surface of the shaft 11 can be an end face with low hardness which can be clinched. The rolling-contact shaft member according to the embodiment of the present invention may be a cylindrical shaft member. In this case, the central portion of the rolling-contact shaft member refers to a portion from the outer peripheral surface to half the depth of the thickness.

Functions and effects of the present invention will be considered based on examples etc. The embodiment of the present invention is not limited to the following examples. A steel material made of SUJ2 was machined to produce a columnar workpiece with a diameter of 11 mm and a length of 22 mm. The workpiece thus produced was then carbonitrided and quenched and also tempered under the heat treatment conditions shown in Table 1 and FIG. 6. Thereafter, the resultant workpiece was polished to produce a shaft member.

FIG. 6 is a diagram showing the heat treatment conditions of Example 1. The carbonitriding and quenching treatments were performed by heating the workpiece at 860° for 4.5 hours in a carbonitriding atmosphere having a carbon potential of 1.2% and an ammonia gas concentration of 3 vol % and then quenching the resultant workpiece to 80° C. by oil. The tempering treatment was performed with the quenched workpiece placed in the heat treatment device 100 shown in FIG. 5. That is, the workpiece W2 was held with its both end faces covered by the jig 102 was immersed in a water-soluble quenchant having a temperature of 25° C., and was inductively heated for 3 seconds at a frequency of 1 kHz and an output of 150 kW. At this time, the cooling solution was supplied through the injection nozzles 106 a at 60 L/m.

A shaft member was produced by a method similar to that of Example 1 except that the conditions for the induction heating (frequency, output, and time) and the flow rate of the cooling liquid supplied through the injection nozzles 106 a in the tempering treatment were changed as shown in Table 1.

A shaft member was produced by a method similar to that of Example 1 except that the quenching treatment and the tempering treatment were performed under the following conditions. FIG. 7 is a diagram showing the heat treatment conditions of Comparative Example 1. Induction through hardening was performed as the quenching treatment. The induction through hardening was performed by inductively heating the workpiece at a frequency of 10 kHz and an output of 23 kW for 15 seconds. Induction tempering was performed as the tempering treatment. The induction tempering was performed by inductively heating the workpiece at a frequency of 2 kHz and an output of 50 kW for 3 seconds.

A steel material made of SCr420 was machined to produce a columnar workpiece with a diameter of 11 mm and a length of 22 mm. The workpiece thus produced was then carbonitrided and quenched and also tempered under the heat treatment conditions shown in Table 1 and FIG. 8. Thereafter, the resultant workpiece was polished to produce a shaft member. FIG. 8 is a diagram showing the heat treatment conditions of Comparative Example 2. The carbonitriding and quenching treatments were performed by heating the workpiece at 850° for 4.5 hours in a carbonitriding atmosphere having a carbon potential of 1.1% and an ammonia gas concentration of 3 vol % and then quenching the resultant workpiece to 80° C. by oil. The tempering treatment was performed by furnace tempering. The furnace tempering was performed by heating the workpiece at 170° C. for 1.5 hours.

A shaft member was produced by a method similar to that of Example 1 except that the quenching treatment and the tempering treatment were performed by the following method. FIG. 9 is a diagram showing heat treatment conditions of Comparative Example 3. Through hardening was performed as the quenching treatment. The through hardening was performed by heating the workpiece at 840° C. for 0.5 hours and then quenching the workpiece to 80° C. by oil. The tempering treatment was performed in a manner similar to that of Example 1 except that the conditions for induction heating (frequency, output, and time) and the flow rate of the cooling liquid supplied through the injection nozzles 106 a were changed as shown in Table 1.

A steel material made of SCr420 was machined to produce a columnar workpiece with a diameter of 11 mm and a length of 22 mm. The workpiece thus produced was then carbonitrided and quenched and also tempered under the heat treatment conditions shown in Table 1 and FIG. 10. Thereafter, the resultant workpiece was polished to produce a shaft member. FIG. 10 is a diagram showing the heat treatment conditions of Comparative Example 4. The carbonitriding and quenching treatments were performed by heating the workpiece at 850° for 5 hours in a carbonitriding atmosphere having a carbon potential of 1.1% and an ammonia gas concentration of 3 vol % and then quenching the resultant workpiece to 80° C. by oil. The tempering treatment was performed by furnace tempering. The furnace tempering was performed by heating the workpiece at 180° C. for 1.5 hours.

TABLE 1 Tempering Cooling Agent Steel Frequency Flow Rate Cooling Agent Species Quenching (kHz) Output (kW) Time (s) (L/min) Species Example 1 SUJ2 Carbonitriding and 1 150 3 60 Water-Soluble Quenching Quenchant Example 2 SUJ2 Carbonitriding and 0.5 210 3 60 Water-Soluble Quenching Quenchant Example 3 SUJ2 Carbonitriding and 0.5 190 3 80 Water-Soluble Quenching Quenchant Example 4 SUJ2 Carbonitriding and 1 140 3 50 Water-Soluble Quenching Quenchant Example 5 SUJ2 Carbonitriding and 0.5 210 5 18 Water-Soluble Quenching Quenchant Example 6 SUJ2 Carbonitriding and 1 100 3 50 Water-Soluble Quenching Quenchant Example 7 SUJ2 Carbonitriding and 1 160 3 50 Water-Soluble Quenching Quenchant Comparative SUJ2 Induction Through Induction Tempering Water-Soluble Example 1 Hardening Quenchant Comparative SCr420 Carbonitriding and Furnace Tempering Quenching Oil Example 2 Quenching Comparative SUJ2 Through Hardening 0.5 80 15 120 Water-Soluble Example 3 Quenchant Comparative SCr420 Carbonitriding and Furnace Tempering Quenching Oil Example 4 Quenching

A Vickers hardness of the outer peripheral surfaces of the shaft members of Examples 1 to 7 and Comparative Examples 1 to 4 and Vickers hardness from the surface to the central portion in the sections of these shaft members were measured with a Vickers hardness testing machine to obtain Vickers hardness distribution from the surface to the central portion. The hardness up to the central portion was measured at intervals of 0.1 mm in the depth direction (radial direction). The maximum gradient Smax and the position x of the maximum gradient Smax were determined based on the hardness distribution in the section. The results are shown in Table 2. Regarding the position of the maximum gradient Smax, “x/d (radius)” was calculated. The calculation results are shown in Table 2.

For the shaft members of Examples 1 to 7 and Comparative Examples 1 to 4, the carbon concentration and the nitrogen concentration in the range from the surface to the depth of 10 μm were measured. The measurement results are shown in Table 2. The carbon concentration and the nitrogen concentration were measured by irradiating each sample with electron beams and measuring intensity of characteristic X-rays generated.

The rolling life of the shaft members of Examples 1 to 7 and Comparative Examples 1 to 4 was measured with a radial type life tester. The measurement results are shown in Table 2. The rolling life was calculated as a relative value to the measured value of Comparative Example 1.

Clinching capability of the shaft members of Examples 1 to 7 and Comparative Examples 1 to 4 was evaluated. This evaluation of clinching capability was performed by a testing method in which the shaft ends are fastened by clinching with a punch. The evaluation results are shown in Table 2. In Table 2, “◯” indicates that the shaft member was deformed by 1.2 mm without cracking when clinched, and “X” indicates that the shaft member was not able to be deformed or cracked when clinched. Manufacturing cost was also calculated as a relative value to the evaluation value of the shaft member of Comparative Example 1. The calculation results are shown in Table 2.

TABLE 2 Hardness Maximum Gradient of Outer Hardness of Carbonitrided Layer Smax Peripheral Internal layer Carbon Content Nitrogen Content Position Rolling Life Clinching Cost Surface (HV) Portion (HV) (wt %) (wt %) (HV/mm) [x/d] (Relative Value) Capability (relative Value) Example 1 750 493 1.15 0.2 179 0.16 1.8 ◯ 0.83 Example 2 763 525 1.2 0.2 200 0.18 2 ◯ 0.83 Example 3 838 546 1.3  0.15 333 0.24 2.5 ◯ 0.9 Example 4 731 498 1.15 0.2 287 0.06 1.5 ◯ 0.85 Example 5 730 589 1.2 0.2 321 0.06 1.2 ◯ 0.87 Example 6 815 558 1.25 0.2 269 0.09 1.2 ◯ 0.82 Example 7 712 477 1.2  0.15 162 0.13 1.1 ◯ 0.85 Comparative 763 252 1 — 1316 0.45 1 ◯ 1 Example 1 Comparative 752 460 1 0.1 323 0.32 1 X 1.1 Example 2 Comparative 768 756 1 — 70 0.16 0.9 ◯ 0.95 Example 3 Comparative 715 720 0.9 0.7 112 0.18 0.8 X 0.95 Example 4

The results in Table 2 show that the shaft members of Examples 1 to 7 have a long rolling life. In particular, the shaft members of Examples 1 to 4 have a significantly long rolling life. The results in Table 2 also show that the ends of the shaft members of Examples 1 to 7 can be clinched.

The rolling-contact shaft member of the present invention is a shaft member whose outer peripheral surface rolling-contacts a mating material and which has a long rolling life. 

What is claimed is:
 1. A rolling-contact shaft member, which is made of high-carbon steel and whose outer peripheral surface serves as a rolling-contact surface that rolling-contacts a mating material, comprising: a carbonitrided layer having a carbon concentration of 1.1 to 1.6 wt % and a nitrogen concentration of 0.05 to 0.6 wt % in a range from a surface to a depth of 10 μm, wherein the rolling-contact shaft member has a Vickers hardness of 700 to 840 HV at the outer peripheral surface, the rolling-contact shaft member has a Vickers hardness of 600 HV or less in its central portion, and a maximum value of an absolute value of a gradient of a change in the Vickers hardness from the outer peripheral surface to the central portion is 100 to 340 HV/mm.
 2. The rolling-contact shaft member according to claim 1, wherein the Vickers hardness at the outer peripheral surface is 720 to 840 HV, and the Vickers hardness in the central portion is 550 HV or less.
 3. The rolling-contact shaft member according to claim 1, wherein the rolling-contact shaft member has a Vickers hardness of 210 to 300 HV at at least one of its end faces.
 4. The rolling-contact shaft member according to claim 2, wherein the rolling-contact shaft member has a Vickers hardness of 210 to 300 HV at at least one of its end faces. 